The invention relates to a method for producing laminated moldings.
Such parts are, for example, used with advantage as inner lining or panelling parts for motor vehicles, but are also widely used in furniture making and in other industrial products.
The subsequently used term "back-embossing" relates to the lamination of random support parts with films or woven or nonwoven fabrics, in which during the initial process sequence a preform is, for example, produced from a thermoplastic material by injection molding or extrusion and whose volume corresponds to that of the finished support part, but whose surface dimensions are smaller than the surface area of the finished support part. In a separate process step, preferably the upper mold half with which the preform is produced is removed from the travel path for the production of the preform and the support part, together with a laminating film, and is completely embossed by means of a second, upper mold half, while retaining the use of the remaining mold means.
The finished embossing associated with material flow processes of the preform requires less contact pressure and lower temperatures than a direct and complete back-injection molding of the lamination in a closed injection mold. It also has the advantage of a more careful and gentle treatment of the possibly pressure and temperature-sensitive laminating material. In addition, the back-embossing permits the production of large-area parts, which is difficult or even impossible when using back-injection molding.
Previously, difficulties have been encountered in back-embossing which need to be overcome, particularly due to the long flow path during the finished or final embossing of the preforms and the associated significant embossing forces, as well as the tendency towards unsatisfactory marginal areas, which are frequently only inadequately shaped.
Long flow paths give rise to increased shear forces between the support material part and the lamination, so that it is not possible to reliably exclude undesired crease formations. There is also a risk of the formation of so-called flow cutting edges with the resulting mechanical weak points in the support part. Difficulties have also been encountered up to now as a result of the need to remove the air between the preform and the lamination during the embossing process, which leads to local bubble formations between the lamination and the support part. Additional problems have been caused in the past by different temperatures and therefore not always clearly defined adhesion conditions between areas of the preform surface and the flow areas during embossing.
The considerable advantages provided by back-embossing for the field of use of interest here must be balanced against the aforementioned difficulties and certainly preponderate. They in particular include the possibility of providing in material-uniform manner laminated moldings and therefore giving rise to clearly defined recycling conditions, which are otherwise scarcely possible with composites. The avoidance of solvent-containing lamination adhesives permits a particularly environmentally protecting and cost-effective manufacture. Lamination and shaping using essentially the same mold significantly simplifies manufacture and further reduces costs.
The problem of the present invention is to so improve back-embossing in the production of laminated moldings that with reduced embossing forces the surface-uniform adhesion of the lamination to the support part is improved and for this purpose the danger of bubble formation must be eliminated and additionally it must be possible to carry out a particular lamination which could not be achieved in the prior art.